The invention relates to a method for the drying and removal of hydrocarbons from gases, especially aromatic hydrocarbons including naphthalene and water, from commercial and natural gases containing hydrocarbons, preferably coal degasifying- and cracking gases, by means of solvents in customary absorption- and desorption apparatuses.
Methods are already known which lower the water vapor dew point of gases, with the goal of having no condensation occurring in subsequent gas treatment, gas transmittance and gas distribution apparatuses, under the given circumstances. The absorption is preferably accomplished by glycol solutions for drying gases for public gas supply or for transport of natural gas over longer distances, with the necessary dew points reaching down to 20.degree. C. (H. Franik, Erdgasaufbereitung, p. 122 ff, VEB Deutscher Verlag fuer Grundstoffindustrie, Leipzig 1964).
If the need exists for still lower dew points, such as, for example, of down to -70.degree. C. in natural gas fractionating plants, it is known that absorbents such as silica gel or molecular sieve can be used (Kohl, Riesenfeld; Gaspurification, p. 571 ff, Gulf Publishing Company, Houston, Tex. 1974).
According to Grosskinsky (Handbuch der Kokereiwesens, Band II, p. 137 ff, Karl-Knapp-Verlag Duesseldorf (1958), benzene hydrocarbons and naphthalenes, contained in gases from coal degasification and oil gasification, can be removed in absorption apparatuses using high-boiling washing oil fractions.
It is further known that benzene hydrocarbons and naphthalenes can be absorbed on active coal, with these components being removed up to a point so that no condensation or sublimation occurs in the subsequent gas treatment, gas transmittance, and gas distribution apparatuses, under the respective pressure- and temperature conditions (Schmidt, Verfahren der Gassaufbereitung, p. 174 ff, VEB Deutscher Verlag fuer Grundstoffindustrie, Leipzig 1970).
Methods are further known, in which the dew point for water vapor, benzene hydrocarbons and naphthalenes is lowered by indirect cooling of the gas flow (Ullrich, Koksofengasaufarbeitung-Naphthalinabtrennung; Erdoel und Kohle (1977) 6, 263 to 267).
These known methods suffer from distinct disadvantages in those instances when the removal of benzene hydrocarbons, naphthalenes and water is to occur in one step and with the lowest possible expenditure of equipment, adjuvants and energy.
The glycol solutions which are used for gas drying, for example di- or triethylene glycol, exhibit a low absorption ability of benzene hydrocarbons and naphthalenes (DE-OS No. 2 449 864). The majority of these components therefore remain in the gas and thus the requisite values for the benzene hydrocarbons and naphthalenes cannot be reached. The operation of a glycol absorption apparatus therefore exhibits a considerable number of operational disturbances, due to the fact that the benzene hydrocarbons accumulate in liquid form, and the naphthalenes in crystal form, during minimal cooling of the gases. In order to be able to effectuate the gas drying with glycol, an active coal arrangement has to be added, resulting in the removal of these components.
The low gas velocity in the absorbers necessitates large apparatuses and considerable amounts of active coal. The desorption additionally requires considerable amounts of vapor, resulting in a very high energy consumption. The result is high investment- and operating costs.
A study of the removal of benzene hydrocarbons and naphthalenes in absorption apparatuses, operating with high-boiling washing oils, reveals the disadvantages that it is impossible to adhere to the necessary final values for gas transmittance and distribution under pressure at usual normal-pressure absorption, and additionally, lowering of the water vapor dew point cannot be achieved.
Using indirect gas cooling in order to lower the dew point is disadvantageous in that a great cooling effort is required to cool the entire gas flow. Furthermore, a fine mist is formed together with the condensation which cannot be separated, even with high performance separators.
It is the object of the invention to increase the absorption ability of the solvent for aromatic hydrocarbons including naphthalene and water, to reduce the investment- and operating costs, to save energy and material, as well as to avoid operational disturbances and to increase safety.